Dissipative ceramic bonding tool tip

ABSTRACT

Methods for making and using dissipative ceramic bonding tool tips for wire bonding electrical connections to bonding pads on integrated circuit chips and packages. The method of using the dissipative ceramic bonding tool tip includes dissipating charge while bonding to avoid damaging delicate electronic devices by a sudden surge of accumulated charge. The method of making the tool tip includes affecting its conductivity so that it conducts electricity at a rate sufficient to prevent charge buildup, but not sufficient to overload the device being bonded. For best results, a resistance in the tip assembly itself should range from 5×10 4  or 10 5  to 10 12  ohms. In addition, the tips must also have specific mechanical properties to function satisfactorily.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/036,579 filed Dec. 31, 2001 now U.S. Pat. No. 6,651,864 issued Nov.25, 2003 entitled “Dissipative Ceramic Bonding Tool Tip” which claimsthe priority benefit of U.S. provisional patent application Ser. No.60/288,203 filed May 1, 2001 entitled “Dissipative Ceramic Bonding Tip”and is also a continuation-in-part of U.S. patent application Ser. No.09/514,454 filed Feb. 25, 2000 now U.S. Pat. No. 6,354,479 issued Mar.12, 2002 entitled, “Dissipative Ceramic Bonding Tool Tip,” which claimsthe priority benefit of U.S. provisional patent application Ser. No.60/121,694 filed Feb. 25, 1999 entitled “Dissipative Ceramic BondingTool Tip.” The contents of the above applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to bonding tool tips in general and moreparticularly to ceramic tool tips for bonding electrical connections.

2. Description of the Prior Art

Integrated circuits are typically attached to a lead frame, andindividual leads are connected with wire to individual bond pads on theintegrated circuit. The wire is fed through a tubular bonding tool tiphaving a bonding pad at the output end. These tips are called capillarytips. An electrical discharge at the bonding tool tip supplied by aseparate Electronic Flame Off (EFO) device melts a bit of the wire,forming a bonding ball. Other bonding tools do not have the center tube,but have a feed hole or other feature for feeding the wire along, asneeded. Some bonding tool tips have no such arrangement for feedingwire, such as bonding tool tips for magnetic disk recording devices,where the wire is insulated and bonded to a magnetic head and then to aflexible wire circuit.

When the bonding tool tip is on the integrated circuit die side of thewire connection, the wire will have a ball formed on the end of thewire, as above, before reaching the next die bonding pad. The ball thencontacts the film formed on the die pad on the integrated circuit. Thebonding tool tip is then moved from the integrated circuit die pad,feeding out gold wire as the tool is moved, onto the bond pad on thelead frame, and then scrubbed laterally by an ultrasonic transducer.Pressure from the bonding tool tip and the transducer, and capillaryaction, causes the wire to “flow” onto the bonding pad where molecularbonds produce a reliable electrical and mechanical connection.

Bonding tool tips must be sufficiently hard to prevent deformation underpressure, and mechanically durable so that many bonds can be made beforereplacement. Prior art bonding tool tips were made of aluminum oxide,which is an insulator that is durable enough to form thousands ofbonding connections. Bonding tool tips must also be designed to producea reliable electrical contact, yet prevent electrostatic dischargedamage to the part being bonded. Certain prior art devices emit one ormore volts when the tip makes bonding contact. This could present aproblem, as a one volt static discharge could cause a 20 milliampcurrent to flow, which, in certain instances, could damage theintegrated circuit or magnetic recording head.

U.S. Pat. No. 5,816,472 to Linn describes a durable alumina bonding tool“without electrically conductive metallic binders” that is therefore aninsulator. U.S. Pat. No. 5,616,257 to Harada describes covering abonding tool electrode with an insulating cap or covering “made of aceramic material” to produce a large electrostatic discharge thatcreates bonding balls of stable diameter. U.S. Pat. No. 5,280,979 toPoli describes a vacuum wafer-handling tool having a ceramic coating“made with a controlled conductivity” to prevent a large electrostaticdischarge.

SUMMARY OF THE INVENTION

The present invention may provide electrically dissipative ceramicbonding tool tips for bonding electrical connections to bonding pads onelectrical devices. In accordance with principles of the presentinvention, the method of using the invention involves an added step ofdissipating electrical charge at a rate sufficiently high to preventcharge buildup, but not high enough to overload the device being bonded.This added step is at least partially counter-intuitive becauseordinarily charge dissipation is avoided so as not to overload thecircuit. Consequently, to avoid damaging delicate electronic devices byany electrostatic discharge, the bonding tool tip is made to conductelectricity at a rate sufficiently high to prevent charge buildup, butnot high enough to overload the device being bonded. In other words, itis desirable for the bonding tool tip to discharge slowly. The tip needsto discharge to avoid a sudden surge of current that could damage thepart being bonded. For best results, a resistance in the tip assemblyitself should range from about 5×10⁴ or 10⁵ to 10¹² ohms. This range ofresistances is adequate no matter the method of characterizing theresistance. The tools may also have a high stiffness and high abrasionresistance so that the tools have a long lifetime. However, bonding tooltips having a low stiffness and low abrasion resistance may also bemade, except that they would have a short lifetime. Possible materialsthat can be used for the bonding tool tips that have a high abrasionresistance and high stiffness include ceramics (electricalnon-conductors) or metals, such as tungsten carbide (an electricalconductor).

In the present invention, bonding tool tips with the desired electricalconduction can be made in at least three different configurations.

First, the tools can be made from a uniform extrinsic semiconductingmaterial that has dopant atoms in the appropriate concentration andvalence states to produce sufficient mobile charge carrier densities(unbound electrons or holes) that will result in electrical conductionin the desired range. For example, the tools can be made frompolycrystalline silicon carbide uniformly doped with boron.

Second, the tools can be made with a thin layer of a highly dopedsemiconductor on an insulating core. In this case, the core provides themechanical stiffness and the semiconductor surface layer providesabrasion resistance and provides a charge carrier path from the tip tothe mount that will permit dissipation of electrostatic charge at anacceptable rate. For example, the tools can be made from a diamond tipwedge that has a surface that is ion implanted with boron.

Third, the tools can be made with a lightly doped semiconductor layer ona conducting core. The conducting core provides the mechanical stiffnessand the semiconductor layer provides abrasion resistance and provides acharge carrier path from the tip to the conducting core, which iselectrically connected to the mount. The doping level is chosen toproduce a conductance through the layer that will permit dissipation ofelectrostatic charge at an acceptable rate. For example, the tools canbe made from a cobalt-bonded tungsten carbide coated with titaniumnitride carbide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vastly enlarged cross-sectional view of a capillary bondingtool tip;

FIG. 2 is a vastly enlarged cross-sectional view of a capillary-typeconstruction of the operating end or tip of a bonding tool;

FIG. 3 is a cross-sectional view of a bottle-neck capillary bonding tooltip;

FIG. 4 is an isometric view of a wedge bonding tool tip;

FIGS. 5 a and 5 b are side and end views, respectively, of the wedgedesign bonding tool tip shown in FIG. 4;

FIGS. 6 a and 6 b are an isometric view and a detailed close-up,respectively, of an apparatus utilized in the wire bonding of asemiconductor integrated circuit chip or other apparatus;

FIG. 7 is a cross-section of an embodiment of FIG. 2 having two layers;

FIG. 8 is a cross-section of an embodiment of FIG. 3 having two layers;

FIG. 9 is a cross-section of an embodiment of FIG. 5 having two layers;

FIG. 10 is a flowchart of a generic method for making a dissipativetool;

FIG. 11 is a flowchart of a first exemplary embodiment of the method ofFIG. 10;

FIG. 12 is a flowchart of a second exemplary embodiment of the method ofFIG. 10;

FIG. 13 is a flowchart of a third exemplary embodiment of the method ofFIG. 10;

FIG. 14 is a flowchart for a method of using the bonding tool tipaccording to the invention;

FIG. 15 is an illustration showing the method of use of a capillarybonding tool tip according to the invention;

FIG. 16 shows sections of the bonding tool whose resistances weremeasured;

FIG. 17 is a table of resistances for two ceramic bonding tools measuredat the points shown in FIG. 16;

FIG. 18 is a schematic representation of the experimental setup used formeasuring the static discharge;

FIG. 19 is a table showing the static decay times measured using theexperimental setup of FIG. 18; and

FIG. 20 is a plot comparing the discharge current at various voltages ofthe ceramic bonding tools to a metal rod.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a typical capillary bonding tool 10 according to theinvention. Such bonding tools 10 can be about one-half inch (12-13 mm)long and about one-sixteenth inch (1.6 mm) in diameter. The bonding tooltip 12 can be from 1 to 8 mils, 2 to 6 mils, or 3 to 10 mils (0.08 to0.25 mm) long. Running the length of the tool itself, but not viewablein FIG. 1, is a tool hole that accommodates a continuously fed length ofgold wire (not shown).

FIG. 2 is a highly enlarged, cross-sectional view of the capillarybonding tool 10 shown in FIG. 1. Only the portion of the bonding tool 10that is shown within the dotted circle in FIG. 1 is shown in FIG. 2.Tool tip 12 has a tool hole 14 which may run the entire length ofbonding tool 10. The wire (not shown) exits the tool tip 12 through anexit hole 18. If a ball is formed on the wire, the ball is seenimmediately adjacent the exit hole 18. The wire may be gold, forexample, but could be made from other conductive metals or mixtures ofconductive metals. The chamfer 16 at the exit hole 18 has at least twopurposes. First, the chamfer 16 accommodates a ball that has been formedat the end of the wire. Second, the chamfer surface 16 allows a smootherlooping of the wire as the bonding tool 10 is moved from the bonding padon an integrated circuit (not shown) to a bonding pad (not shown) on alead frame (not shown) of an integrated circuit assembly (not shown).The inner diameter of the bonding tool tip 10 may be about 1.5 times thewidth of the wire being fed through it. For example the inner diametermay be 1.3 or 1.4 to 1.6 microns.

Although the size of the bonding tool 10 may change according to thesize of the component being manufactured, the diameter of the tool tip12 may remain essentially the same.

FIG. 3 shows an alternative embodiment of a bonding tool 10 havingsimilar features, such as the tool hole 14, chamfer surface 16, and exithole 18. This bonding tool tip, named a bottle-neck capillary tip, isprovided for narrower bond situations where the bonding pitch (distancebetween the centers of the bonding pads) is small. Bonding tool tips andthe bonding pitch tend to get smaller as the dimensions of integratedcircuits get smaller, or as the number of circuits on a chip getslarger, while the die area remains more or less constant.

FIG. 4 shows still another type of bonding tool 10, called a wedge tool,having end 14, raised portion 16, and grooves 18. The FIG. 4 embodimentof bonding tool 10 can be used for disk drive bonding where it is usedto capture the insulated wire, lay it on the head of bonding tip 12 andultrasonically bond it to a part of the disk drive system, for example,or other device being bonded. Bonding tool 10 may also be used with anintegrated circuit die mounted on a lead frame (not shown). When bondinga magnetic recording head or integrated circuit dies the wires from themagnetic recording head or integrated circuit die may not be connectedfrom the die directly to connections in an integrated circuit package,but from the magnetic recording head or integrated circuit die to a leadframe, as is well-known to skilled practitioners in the art. Thecomposition of the lead frame may be different than the composition ofthe integrated circuit package. The tip 12 of the bonding tool 10 ofFIGS. 5 a and 5 b accommodates the different physical attributes ofdifferent integrated circuit lead frames. The grooves 18 in FIGS. 4, 5 aand 5 b frictionally hold the pad being bonded in place duringultrasonic bonding. The grooves 18 are typically “v” shaped but othershapes such as cylindrical also work. The size of the grooves 18 and/ordie area may be kept essentially constant despite differences in size ofthe component being worked on. The width of the grooves 18 may beapproximately the same or slightly smaller than the diameter of the wirebeing bonded. In an embodiment, the grooves 18 are 1 to 30 microns wideand 1 to 30 microns deep. The grooves 18 may cut through the entiredepth of the raised portion 16, which may also be 1 to 30 microns deep.In an embodiment, raised portion 16 is 6 to 7 microns deep, grooves 18are 2.5 to 4.5 microns deep, raised portion 16 is 100 to 150 micronswide. Raised portion 16 and end 14 may be 8 to 35 or 40 microns wide.Although FIGS. 4, 5 a, and 5 b show two grooves 18 forming a cross thebonding tool tip 12 may have just one groove or a mesh of intersectingand/or parallel groves. Although the grooves 18 are illustrated as beingperpendicular they may be at any angle with respect to one another.

FIG. 6 a illustrates a typical wire bonding machine 60 for use inbonding wire leads in magnetic disk drive units. Shown within the dottedcircle is the bonding tool 10. The bonding tool 10 is mounted to an arm66 that can be moved in the desired directions by the apparatus of wirebonding machine 60. Such a machine is available as Model 7400 from theWest Bond Company in Anaheim, Calif.

Typical bonding tool tips available on the market today are made of aninsulator of alumina (Al₂O₃), sometimes termed aluminum oxide, ruby, orsapphire, which are very hard compounds that have been used successfullyon commercial machines. Wire bonding tool tips made of alumina, ruby, orsapphire have a reasonably long lifetime. In the prior art, to ensurethat the tool tip is an insulator, no conductive binders are used inthese bonding tool tips. However, as stated previously, a problem hasexisted that an electrostatic discharge from the bonding tool makingcontact with the bonding pad of the circuit can damage the very circuitit is wiring.

In accordance with principles of the present invention, to avoiddamaging delicate electronic devices by this electrostatic discharge,bonding tool tip 12 should conduct electricity at a rate sufficientlyhigh to prevent charge buildup, but not high enough to overload thedevice being bonded. It has been determined that the bonding tool 10 mayhave an electrical conductance greater than one ten-billionth of a mho(i.e. >1×10⁻¹² reciprocal ohms (Ω⁻¹) of power) and its electricalconductivity may be less than one one-hundred thousandth of a mho (i.e.<1×10⁻⁵ Ω⁻¹). The resistance should be low enough that the material isnot an insulator that does not allow charge dissipation, and high enoughthat it is not a conductor allowing a current flow that is damaging tothe device being bonded. For best results, a resistance in the tipassembly itself should range from 5×10⁴ or 10⁵ to 10¹² ohms. Forexample, today's magnetic recording heads are damaged by 5 milliamps ofcurrent. In an embodiment that may be used with magnetic recordingheads, no more than 2 to 3 milliamps of current should be allowed topass through the bonding tool tip 12 to the head.

In an embodiment, to achieve high stiffness and high abrasionresistance, ceramics (electrical non-conductors) or metals, such astungsten carbide (an electrical conductor) are used. The bonding tooltip of this embodiment may have a Rockwell hardness of about 25 orabove, preferably of about 32 or above. The tip needs to be able to lastfor at least two bondings.

In the present invention, bonding tool tips with the desired electricalconduction can be made in at least three different configurations.

First, the tools can be made from a uniform extrinsic semiconductingmaterial that has dopant atoms in the appropriate concentration andvalence states to produce sufficient mobile charge carrier densities(unbound electrons or holes) that will result in electrical conductionin the desired range. For example, polycrystalline silicon carbideuniformly doped with boron can give the desired range of conductivity.Preferably the amount of boron used is 5-7% by weight of thepolycrystalline silicon carbide.

Second, the tools can be made by forming a thin layer of a highly dopedsemiconductor on an insulating core. For example, a diamond tip wedgemay have a surface that is ion implanted with boron or have a surfacethat is a doped ceramic. In this case the core provides the mechanicalstiffness and the semiconductor surface layer provides abrasionresistance and provides a charge carrier path from the tool tip 12 tothe mount (not shown), which will permit dissipation of electrostaticcharge at an acceptable rate. The conductance of the semiconductorsurface layer should be about 10⁸-10⁹ Ω⁻¹.

Third, the tools can be made by forming a lightly doped semiconductorlayer on a conducting core, for example, a cobalt bonded tungstencarbide core coated with titanium nitride carbide. The conducting coreprovides the mechanical stiffness and the semiconductor layer providesabrasion resistance and provides a charge carrier path from the devicebeing bonded to the conducting core, which is electrically connected tothe mount. The doping level is chosen to produce a conductance throughthe layer that will permit dissipation of electrostatic charge at anacceptable rate. The conductivity of the semiconductor surface layershould be about 10⁷-10⁸ Ω⁻¹.

FIGS. 7, 8 and 9 illustrate the two-layered structure of the last twoconfigurations. This structure is not intended to be specific to thetype of tool tip. Rather, it could be used for any bonding tool tip.Layers 71, 81, and 91 could be 100-1000 Angstroms thick, for example. Inthe second and third configurations, the outer layers are labeled 71,81, and 91 and the cores are labeled 72, 82, and 92. In the secondconfiguration, mentioned above, layers 71, 81, and 91 are highly dopedsemiconductor and the cores 72, 82, and 92 are insulators. In the thirdconfiguration, mentioned above, layers 71, 81, and 91 are lightly dopedsemiconductor and the cores 72, 82, and 92 are conductors. Nosignificance should be attached to the relative thickness or scale ofthe portions of the layer 71, 81, and 91, which may or may not have auniform thickness.

Dissipative tools can be manufactured by any of several methods.

FIG. 10 illustrates a generic method 1000 for manufacturing dissipativetools. The process of creating a ceramic part may start with a powderhaving the same or a similar composition as desired in the ceramic partto be created. The quality of the ceramic component may be influenced bythe quality of the ceramic powder used. To ensure quality, the ceramicpowder may be tested and processed multiple times. The purity,concentration of agglomerations, and particle size of the ceramic powdermay be monitored. The powder may be milled (e.g., attrition milled,balled milled, or turbo milled). The milling operation refines theparticle size of the ceramic powder before process 1000 begins. In step1002 a material, which may initially be a powder, is formed having thedesired composition. The material is next shaped and sized in step 1004into a form appropriate for the tool. The material may be furthertreated in step 1006 to affect or impart desired mechanical, chemicaland/or electrical properties. Depending upon the embodiment, steps 1002,1004, and 1006 may be performed simultaneously as part of one process.Since the properties of the material depend upon the process of makingand the materials used for making the composition, parts or all of step1006 may be performed before step 1004. In optional step 1008 thematerial is sized to tolerance. In optional step 1010 the layering isformed. In optional step 1012 the material is further treated to impartdesired properties to the layers or affect the desired properties of thelayers.

FIGS. 11-13 show three examples of the method of FIG. 10.

FIG. 11 shows method 1100, which includes mixing, molding and sinteringreactive powders of, for example, alumina (Al₂O₃), zirconia (Zr₂O₃),iron oxide (FeO₂), or titanium oxide (Ti₂O₃).

In general, sintering may involve the densification of powder compactsat a temperature below the melting point of the powder. The shrinkageoccurs as the pores between the particles decrease in size until theyare eliminated. The driving force of the sintering process is thereduction of surface energy. During the sintering of two sphericalparticles, for example, the inter-particle contact areas will increaseas the growth into a neck between the particles increases. There arethree basic stages involved with the sintering process. In the firststage, the material between the particles moves outward by viscous flow,plastic flow or volume diffusion and is deposited on the neck area. Thedistance between the particle centers decreases and shrinkage occurs. Ifthe material is transported from the circumference into the neck byevaporation-condensation or surface diffusion then there is noshrinkage. In the second stage, the growing necks merge, the originalparticle structures disappear and are replaced by polycrystalline bodieswith an inter-granular pore network along grain boundary edges. Thegrain growth can occur by the movement of grain boundaries towards theircenters of curvature. In the third stage the grain growth continues;pores become closed at grain corners and further densification occurs asthe pores shrink. If the grain boundaries are sufficiently curved, theycan move over the pores leaving them isolated in the grains. The processof further shrinkage may be slow once the pores are within the grains.

In step 1102 fine particles (e.g., a half of a micron in size) of thedesired composition are mixed with organic and inorganic solvents,dispersants, binders, and sintering aids. The solvents could be Yttriumor H₂O, for example. The binder and/or the sintering aids could be anyof, any combination of, or all of ceria, magnesia, yttria, boron, carboncolloidal silica, alumina solvents, ethyl silicate, any phosphate, anyrare earth metal oxide, or yttrium, for example. In step 1104 the mix ismolded into oversize wedges. The pieces are carefully dried, and heatedslowly in step 1106 to remove the binders and dispersants and thenheated in step 1108 to a high enough temperature so that the individualparticles sinter together into a solid structure with low porosity. Theslow heating can be done over three to eight hours at a rate of 50° C.to 200° C. every 15 minutes, for example, in an atmosphere of 500° C. or1000° C. to 2500° C. for 3 to 24 hours, so as to obtain low porosity,and to obtain homogeneity. The sintering can occur at 4000° C., forexample. The heat-treating atmosphere is chosen to facilitate theremoval of the binder at a low temperature and to control the valence ofthe dopant atoms at the higher temperature and while cooling. The lowporosity can be ensured by keeping the grain size less than about half amicron. Next, in step 1110, the solid structures are allowed to coolpreferably over a period of one to two hours. After cooling, in optionalstep 1112, the pieces may be machined or otherwise sized to achieve therequired tolerances. In optional step 1114 the pieces may then betreated to produce the desired surface layer by ion implementation,vapor deposition, chemical vapor deposition, physical deposition,electroplating deposition, neutron bombardment, or combinations of theabove. The pieces may be subsequently heat treated in optional step 1116in a controlled atmosphere to produce desired layer properties (e.g.,the desired hardness and resistivity) through diffusion,recrystallization, dopant activation, or valence changes of metallicions.

In an example, in step 1104 silicon nitride or zirconia ceramicmaterials could be fabricated by firing a powder compact at a suitabletemperature until agglomeration of the particles occurs with a decreasein the surface area and porosity of the compact. This process mayinvolve chemical reactions, crystal growth and/or the formation ofliquid phases and solid state diffusion. An untreated silicon nitrideceramic powder is typically in the alpha phase. The sintering process ofstep 1106 involves heating the ceramic powder to +2000° C. to convertthe powder to the preferred beta-Si₃N₄ state. The beta-Si₃N₄ state hasthe high thermo-mechanical properties suitable for high temperatureapplications such as resistive heating. Silicon nitride is verydifficult to sinter because it has very strong directional covalentbonds. Although silicon nitride may be at least partially sinteredwithout adding sintering aids, the ceramic powder may not completelyturn from the alpha phase to the beta-Si₃N₄ phase during the heatingprocess without the sintering aids. Sintering aids of rare earth oxidesand other oxides may act as nucleating agents for the Si₃N₄ powders tonucleate the formation of grains. Yttria (Y₂O₃) and Aluminum Oxide(Al₂O₃) may be used as the sintering aids although other sintering aidswill also work.

In another example, silicon carbide, zirconia, or silicon nitride couldbe used for the bonding tip 12. Although silicon nitride does not needmuch preparation before it enters the sintering stage of step 1106,silicon carbide and zirconia have two phases that can exist that mayaffect the quality of the finished product. Silicon nitride has twophases, alpha and beta-Si₃N₄, a hexagonal structure, and can be used tomake a polycrystalline ceramic. Similarly, zirconia exists as amonoclinic crystal at room temperature and inverts to a tetragonal phaseabove 1200° C. In other words, zirconia has a low temperature monoclinicstate and a high temperature tetragonal state. The silicon nitride betaphase and the tetragonal zirconia crystal have the higher strengthproperties of their two respective phases but some stabilizers should beadded in step 1102 in order to induce silicon nitride and zirconia toremain in their beta phase and tetragonal phase, respectively, duringthe cooling step 1110. For example, a stabilizer such as magnesium oxidemay be added in step 1102 to prevent the transformation upon cooling instep 1110. The addition of yttria in step 1102 yields an extremely finegrained (less than 1 micron) microstructure known as tetragonal zirconiapolycrystal (TZP).

The process of mixing in the additives during step 1102 to achieve thehigher strength phase is called forming the green body.

There are several other types of sintering processes that can be used tomanufacture the bonding tool tip. In reaction bonding sintering, in step1106 the green body is placed in a chamber where it is heated andinfiltrated with a reacting gas to form a compound. The process ofreaction bonding silicon nitride to form a silicon nitride bonding tooltip involves taking a silicon green body between steps 1106 and 1108 andreacting the body to a gas of hydrogen and nitrogen to form Si₃N₄.Exposing the green body to the hydrogen and nitrogen gas is commonlyknown as nitriding. The body is nitrided in the gas starting at 1150° C.and slowly increasing the temperature to 1420° C. The resulting productis a mixture of alpha and beta silicon nitrides with 18 to 25% porosity.The original dimensions of the silicon compact remain virtuallyunchanged during the nitriding. The bonding tool tip can be machinedafter partial nitriding in step 1112. Reaction bonding can be relativelycheap.

When using hot press sintering to form a bonding tool tip, a ceramicpowder is placed in a die and then it is compressed at a high pressurewhile the powder is heated in step 1104. When working with siliconnitride powers, the powder is hot pressed with a suitable oxide additivein a graphite die and it may be heated by induction, for example, to1700° C. to 1800° C. to give a fully dense high strength beta-siliconnitride. Diamond machining follows the hot pressing.

When using Hot Isostatic Pressing (HIP) to form the bonding tool tip, instep 1104 the powder is placed in an evacuated pressure vessel. Thevessel will simultaneously heat and isostatically press the materialwith an inert gas with pressures as high as 310 MPa (45,000 psi) andtemperatures up to 2000° C. The powder is simultaneously heated andisostatically pressed by inert gas pressure until densified.

FIG. 12 illustrates method 1200 of hot pressing reactive powders. Fineparticles (e.g., a half of a micron in size) of the desired compositionare mixed in step 1202 with binders and sintering aids and then pressedin a mold in step 1204 at a high enough temperature to causeconsolidation and binding of the individual particles into a solidstructure (e.g., 1000° C. to 4000° C., preferably 2000° C.) with lowporosity (e.g., having grain size of less than half a micron in size).The hot pressing atmosphere is chosen to control the valence of thedopant atoms. After cooling and removal from the hot press in step 1206,the pieces may be machined or otherwise sized to achieve the requiredtolerances in step 1208. The pieces may then be treated in optional step1210 to produce the desired surface layer (e.g., 100 to 1000 Angstromsthick) by ion implantation, vapor deposition, chemical vapor deposition,physical deposition, electo-plating deposition, neutron bombardment orcombinations of the above. In optional step 1212 the pieces maysubsequently be heat treated (e.g., 2000° C. to 2500° C. for 3 to 5minutes) in a controlled atmosphere to produce the desired layerproperties through diffusion, recrystallization, dopant activation,and/or valence changes of metallic ions.

FIG. 13 illustrates method 1300 of fusion casting. Metals of the desiredcomposition are melted in step 1302 in a non-reactive crucible then castinto an ingot. The ingot is then rolled in step 1304, extruded in step1306, drawn in step 1308, pressed in step 1310, heat treated (e.g., at1000° C. or 500° C. to 2500° C. for one to two hours) in step 1312 in asuitable atmosphere, and chemically treated in step 1314. The rolling1304, extruding 1306, drawing 1308 and pressing 1310 steps shape the tipand the heat treatment 1312 and chemical treatment 1314 steps are foraffecting or imparting the mechanical and electrical properties such asthe hardness and resistivity. The pieces are then optionally machined orotherwise sized to achieve the required tolerances in step 1316. Themetallic pieces are then optionally heat treated to produce the desiredsurface layer by vapor deposition, chemical vapor deposition, physicaldeposition, electo-plating deposition, or combinations of the above instep 1318. The pieces may be subsequently heat treated (e.g., at 4000°C. for three to four hours) in a controlled atmosphere to produce thedesired layer properties through diffusion, recrystallization, dopantactivation, or valence changes of metallic ions in step 1320.

Although steps 1008, 1112, 1208, and 1316; 1010, 1114, 1210, and 1318;and 1012, 1116, 1212 and 1320 share similar descriptions they are givendifferent labels because the details of how to best carry out thesesteps may be partly dependent upon the details of the preceding steps.

In the three methods above the heat-treating, hot pressing, andcontrolled atmospheres are preferably primarily an inert gas such asnitrogen using a nitrogen-based furnace.

The green body for the bonding tool tip can be formed by using a varietyof other methods of casting high temperature ceramics such as injectingmolding, cold isostatic, extrusion, slip casting, Hot Isostatic Pressing(HIP), and gelcasting

Injection molding can be used with all types of ceramics. The featuresbasic to injection molding are that the powder is placed in athermosetting polymeric binder, is injected into a mold where it hardenswith time, and then is ejected from the mold. A concern with injectionmolding is that the de-waxing or removing the resin should be donewithout degrading the surface of the green body.

When using slip casting, a slip may be made of water and the ceramicpowder. The slip is cast into an absorbent mold. The casting rate isdependant on the pressure applied to the slip cast and the castthickness. The geometry of the casting surface may also affect thecasting time.

In extrusion, a feedrod for coextrusion is formed from the compoundedmaterial, which may have of a silicon nitride-filled core with acladding of boron nitride-filled material. The feedrod is then extrudedthrough a heated die to form fine filaments.

In dipcoating a single component filament (such as a siliconnitride-filled polymer) is pulled through a slurry of boron nitridewhich dries to form the cell boundary material.

Gelcasting is a ceramic-forming process for making high-quality,complex-shaped ceramic parts. Gelcasting can be used for making bondingtool tips 12 with any of the ceramic powders mentioned in thisspecification. Gelcasting involves mixing ceramic powders in apolymerizable aqueous monomer solution that is then gelled in a mold.The cast body will be both homogeneous in its chemistry and have acertain density, resulting in the material properties (e.g., hardnessand resistivity) being constant throughout the body and the drying andsintering processes having uniform volume changes. Using Gelcasting, thecasting time from design to final fired part can be one week.

Layers 71, 81, and 91 of bonding tool tip 12 may be made from severalcompositions of matter. A formula for dissipated ceramic may includealumina and zirconia and/or other elements. This mixture is bothsomewhat electrically conductive and mechanically durable. The tip of abonding tool is coated with this material or can be made completely outof this material. The tip may be wedge-shaped or circular-shaped asshown and described in the earlier FIGS. 1 to 5, for example.

One actual sample was constructed with the following elements: Iron,Oxygen, Sodium, Carbon, Zirconium, Silicon, Aluminum, Yttrium.

While the range of alumina could extend from 15% to 85% and the range ofzirconia from 15% to 85%, in one embodiment the sample included aluminaat 40% and zirconia at 60%.

FIG. 14 is a flowchart for a method of using the invention. In optionalstep 1402 an initial potential is established between the bonding tooltip and the item being bonded that is sensitive to electrical discharge.Although not necessary, establishing a potential may give the user someadditional control over how the tip discharges. Establishing a potentialmay involve establishing an electrical connection or grounding the leadframe, individual leads on the integrated circuit and/or the individualbond pads on the integrated circuit. In step 1404 the bonding tool tipis placed in contact with the items being bonded together to hold themin place. In step 1406 the bond is formed. Steps 1404 and 1406 may beperformed simultaneously as part of the same step. In step 1408 thecharge is dissipated. This step may be performed simultaneously withsteps 1404 and 1406. It is important that this step be performedwhenever the tip and the electrostatic discharge sensitive component arein contact to prevent a discharge.

For example, in the case of a capillary tip the wire is fed through thetubular bonding tool tip prior to placing it in contact with the itemsbeing bonded. Then an electrical discharge at the bonding tool tip issupplied by a separate EFO device to melt a bit of the wire, forming abonding ball. The ball then makes intimate contact with the film formedon the die pad on the integrated circuit, initiating the dissipation ofcharge. The bonding tool tip is then moved from the integrated circuitdie pad, with gold wire being fed out as the tool is moved, onto thebond pad on the lead frame, and then scrubbed laterally by an ultrasonictransducer. Pressure from the bonding tool tip and the transducer, andcapillary action, ‘flows’ the wire onto the bonding pad where molecularbonds produce a reliable electrical and mechanical connection whilestill dissipating charge. In this example the bonding, the contactbetween the bonding tool tip and the electrostatic discharge sensitiveintegrated circuit, and the dissipation all occur essentiallysimultaneously.

FIG. 15 shows a capillary bonding tool 10 being used to bond wire 1502to pad 1504. Ball 1506 will be used to bond wire 1502 to the next point.The bonding joint 1508 was formed with a ball similar to 1506. Thedifference between this method of use and the prior art is primarily inthe dissipation of charge from the bonding tool 10.

The bonding tool tip 12 of the present invention could be used for anynumber of different types of bonding. Two examples are ultrasonic andthermal bonding.

FIG. 16 shows sections of the bonding tool 10 having end 1602 and points1604-1614. Point 1604 is 1 inch from end 1602 whose resistances weremeasured. Points 1604-1614 are each one inch apart.

Two ceramic rods #1 and #2 (not shown), were used as the base materialfor ceramic wire bonding tool tips 12 to form bonding tools 10 accordingto the invention. The two rods each had a diameter of approximately 0.07inches. The point-to-point resistances along both of the rods weremeasured from the end of the bonding tool tip to various points alongthe tool tip at 10 and 100 volts. The resistance at each voltage wasmeasured six times, each time from end 1602 to a different one of points1604-1614 to obtain measurements of a 1, 2, 3, 4, 5, and 6 inch section,respectively, that starts at end 1602.

FIG. 17 is a table of resistances for two ceramic bonding tools measuredat the points shown in FIG. 16. As shown in the table and as discussedin the preceding paragraph, the resistances were measured at 1, 2, 3, 4,5, and 6 inches at 10V and 100V. The two bonding tools hadpoint-to-point resistances that varied between 1.8×10⁸ Ω and 1.9×10⁹ Ω.After measuring the resistances according to FIG. 16 the staticdischarge was measured.

FIG. 18 is a schematic representation of the experimental setup used formeasuring the static discharge having bonding tool 10, clamp 1802,voltmeter 1804, current probe 1806, oscilloscope 1808, and ElectroStaticDischarge (ESD) simulator 1810.

The static discharge was measured by charging bonding tool 10 andmeasuring the time required for the charge to dissipate. The charge wasassumed to have dissipated once the current from the bonding tool 10 toground dropped off significantly from its initial value (e.g., thecurrent was less than 10% of its initial value). The current wasmeasured from the bonding tool 10 when it was charged and grounded. Thebonding tool 10 was held in insulative clamp 1802 on a ring stand (notshown), charged to a known voltage with ESD simulator 1810. The voltagewas verified using voltmeter 1804, and then the bonding tool 10 wasgrounded. The current moving through the ground wire was measured withcurrent probe 1806 connected to oscilloscope 1808. Ten measurements weremade at each voltage level on the bonding tool 10. Using the setup ofFIG. 18 the bonding tool 10 can be charged and discharged successively.The rise and fall of the current is plotted by the trace on oscilloscope1808 which allows the discharge time of several successive cycles ofdischarging to be viewed and measured graphically.

The voltmeter 1804 could be, but is not limited to, a TREK model 341non-contact voltmeter. The current probe 1806 could be, but is notlimited to, a CT-1 current probe. Oscilloscope 1808 could be, but is notlimited to, a Tektronics TDS 520A Digital Oscilloscope. The ESDsimulator 1810 could be, but is not limited to, a KeyTech MZ-15.

FIG. 19 is a table showing the static decay times measured using theexperimental setup of FIG. 18. The static decay from 1000 volts to 10volts was also measured on both rods, #1 and #2. The static decay timesvaried between 0.1 and 0.5 seconds, or more precisely between 0.12 and0.48 seconds, indicating how quickly the charge dissipate. The decaytime is the product of the resistance times the capacitance. Using thedata of the tables of FIGS. 17 and 19 an estimate of the capacitance asa function of position associated with the bonding tool 10 can be made,indicating how much charge may build up in bonding tool 10.

FIG. 20 is a plot comparing the discharge current at various voltages ofthe ceramic bonding tools to a metal rod. The averages of the current ateach voltage level are plotted in FIG. 20. One of the bonding tools (#1)was measured at five different voltages, and the other bonding tool (#2)was measured at two voltage levels to verify the discharge currents. Thedata points representing the two bonding tool tips are marked usingsquares for one tool tip and triangles for the other. The data pointsrepresenting the metal rod are marked with diamonds. The resistanceassociated with this measurement is around 1×10⁵ Ω or more preciselybetween about 7.5×10⁴ Ω and 2.8×10⁵ Ω. The current represents thedischarge rate. Clearly the bonding tools discharge at a slower ratethan the metal rod.

While the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, modifications may be made without departing fromthe essential teachings of the invention.

1. A device comprising: a tip of a bonding tool having a dissipativematerial for use in wire bonding machines for connecting leads tointegrated circuit bonding pads, wherein the tip has a static dischargetime between 0.1 and 0.5 seconds.
 2. A device comprising: a bonding tooltip having an electrically dissipative ceramic for use in capillarywedge-type wire bonding machines for connecting leads to integratedcircuit bonding pads.
 3. A method of using a bonding tool tip,comprising: providing an electrically dissipative bonding tool tip;bonding a material to a device; allowing an essentially smooth currentto dissipate to the device, the current being low enough so as not todamage said device being bonded and high enough to avoid a build up ofcharge that could discharge to the device being bonded and damage thedevice being bonded.